Classes of materials used in medicine - Biomedical Engineering Desk Reference - page 260

(Rootare and Craig, 1978; LeGeros, 1965). The sub-
stitution of electronegative anions such as fluorine and
chlorine for (OH) has also been reported to alter the
lattice parameters of the material (e.g., Young and Elliot,
1966; Kay et al. , 1964; see also Elliot, 1994).
HA may be processed as a ceramic using compaction
(die pressing, isostatic pressing, slip casting, etc.)
followed by solid-state sintering (discussed earlier in this
section). When reporting methods for the production
and sintering of HA powders it is very important to ad-
equately characterize the morphology of the product
including the surface area, particle size distribution,
mean particle size, and physical appearance of the pow-
ders, since this will greatly influence the handling and
processing characteristics of the material (Best and
Bonfield, 1994). There is great deal of variation in the
reported mechanical performance of dense HA ceramics,
dependent on phase purity, density and grain size, but the
properties cited generally fall in the range shown in Table
3.2.10-9 .

400 m/sec and a temperature within the arc of 20,000 K.
The ceramic powder is suspended in the carrier gas and
fed into the plasma where it can be fired at a substrate.
There are many variables in the process including the gases
used, the electrical settings, the nozzle/substrate separa-
tion and the morphology, particle size, and particle size
distribution of the powder. Because of the very high
temperatures but very short times involved, the behavior
of the HA powder particle is somewhat different than
might be predicted in an equilibrium phase diagram.
However, according to the particular conditions used, it is
likely that at least a thin outer layer of the powder particle
will be in a molten state and will undergo some form of
phase transformation, but by careful control of the oper-
ating variables the transformed material should represent
a relatively small volume fraction of the coating and the
product should maintain the required phase purity and
crystallinity (Cook et al. , 1998; Wolke et al. , 1992).
A number of factors influence the properties of the
resulting coating, including coating thickness (this will
influence coating adhesion and fixation the agreed op-

timum now seems to be 50-100 m m; Soballe et al. , 1993
and de Groot et al. , 1987), crystallinity (this affects the
dissolution and biological behavior; Klein et al. , 1994a, b;
Clemens, 1995; Le Geros et al. , 1992), biodegradation
(affected by phase purity, chemical purity, porosity,
crystallinity), and adhesion strength (these may range
between 5 and 65 MPa (97)).
The mechanical behavior of calcium phosphate ce-
ramics strongly influences their application as implants.
Tensile and compressive strength and fatigue resistance
depend on the total volume of porosity. Porosity can be in
the form of micropores ( < 1 m m diameter, due to in-
complete sintering) or macropores ( > 100 m m diameter,
created to permit bone growth). The dependence of
compressive strength (s c ) and total pore volume ( V p )is
described in de Groot et al. (1990) by:

d

Calcium phosphate coatings

The clinical application of calcium phosphate ceramics is
largely limited to non-major-load-bearing parts of the
skeleton because of their inferior mechanical properties,
and it was partly for this reason that interest was directed
toward the use of calcium phosphate coatings on metallic
implant subtrates. A very good review of techniques for
the production of calcium phosphates was given by Wolke
et al. in 1998. Many techniques are available for the de-
position of HA coatings, including electrophoresis, sol-gel
routes, electrochemical routes, biomimetic routes, and
sputter techniques, but the most popular commercial
routes are those based on plasma spraying. In plasma
spraying, an electric arc is struck between two electrodes
and stream of gases is passed through the arc. The arc
converts the gases into a plasma with a speed of up to

s c ¼ 700 exp 5 V p ð in Mpa Þ (3.2.10-3)

where V p is in the range of 0-0.5.
Tensile strength depends greatly on the volume frac-
tion of microporosity ( V m ):

Table 3.2.10-9 Typical mechanical properties of dense
HA ceramics

s t ¼ 220 exp 20 V m ð in MPa Þ (3.2.10-4)

3.156 g cm 3

Theoretical density

The Weibull factor ( n ) of HA implants is low in
physiological solutions ( n ¼ 12), which indicates low
reliability under tensile loads. Consequently, in clinical
practice, calcium phosphate bioceramics should be used
(1) as powders; (2) in small, unloaded implants such as in
the middle ear; (3) with reinforcing metal posts, as
in dental implants; (4) as coatings (e.g., composites); or
(5) in low-loaded porous implants where bone growth
acts as a reinforcing phase.

Hardness 500-800 HV, 2000-3500 Knoop

Tensile strength 40-100 MPa

Bend strength 20-80 MPa

Compressive strength 100-900 MPa

Approx. 1 MPa m 0.5 Fracture toughness

Young 0 s modulus 70-120 GPa